The present invention belongs to the field of density measurement and relates to a fluid for measuring the density or related properties (e.g., volume, specific gravity) of solid bodies, as well as a method and apparatus for performing the measurement.
The density of solid bodies is commonly determined by weighing the body in air (dry weight) and subsequently weighing the same body in a fluid (wet weight), normally in water. The density, specific gravity, or volume of the solid body is then calculated in an essentially known manner from the dry and wet weight values. Various versions of essentially the same method are known where, e.g., an entire balance is immersed in a fluid, or the container that holds the fluid is weighed while the solid body under test is suspended from a fixed point into the fluid.
The process (in any of its various forms) is known to work well for solid bodies of higher density than water. However, a complication occurs with bodies of lesser density, because such bodies will float on the surface of the water. This has led to a method of measuring solid bodies of lesser density in an alcoholic medium instead of water. A closer examination of the measuring results achieved with alcohol has shown, however, that accuracy and reproducibility are unsatisfactory for at least a number of practical applications. Alcohol has a high volatility and, therefore, alcoholic mixtures have a variable density.
Distilled water can be handled without problems as a medium for density determinations. However, it has a relatively strong tendency to absorb gases, particularly CO2, combined with a relatively week tendency of wetting the surfaces of immersed objects. It is possible for gas bubbles to attach themselves to rough surfaces and falsify the results. Methods are known for degassing fluids by means of ultrasound but this involves a need for additional equipment, and it adds a certain amount of energy to the fluid with the result of an increase in temperature. Other fluids are known to be usable for density determinations, but they can overcome no more than a part of the aforementioned disadvantages. For example, the fluid known as FC 40 has good wetting properties, but with a density of 1.8 g/cm3, it is suitable only for measurements of solid bodies of high density or for methods of measuring the density of floating bodies. In addition, it is fairly volatile, which causes its density to be variable.
It is therefore the object of the present invention, to provide a fluid, a method, as well as an apparatus, by which the density and related properties of solid bodies can be measured with a higher degree accuracy and reproducibility.
Based on extensive serial experiments performed by the inventor(s), it was found that the aforementioned object of the invention can be met with a fluid with the properties that
a) The density of the fluid is smaller than the density of water;
b) The surface tension is significantly smaller than the surface tension of water;
c) The rate of evaporation is slower than the evaporation rate of water, the vapor pressure of the fluid being smaller than the vapor pressure of water by at least a factor of 2;
d) The water absorption, i.e., the hygroscopicity of the fluid is less than 1%.
Having knowledge of this advantageous combination of properties, a person skilled in the art will require little effort to find a fluid that meets the stated characteristics.
In principle, a low level of volatility of the fluid is desirable. Alcohol, in particular, does not meet this requirement. According to the invention, it is advantageous to use a fluid with a vapor pressure that is smaller than the vapor pressure of water by at least a factor of 4.
Of course, the scope of the invention also covers the use of mixtures of fluids. To obtain good results in this case, it is important to use an approximately azeotropic mixture. By definition, this is a mixture in which all components have essentially the same boiling point, so that there is no separation or change in the composition of the mixture due to different amounts of evaporation over an extended period of time, which would cause a change in the properties of the mixture. Nearly azeotropic mixtures in a variety of compositions have been proposed, e.g., as replacements for fluids containing fluorocarbons, so that a person skilled in the art and equipped with a knowledge of the present invention will have a choice of fluids available.
One problem in using fluids other than water lies in the naturally occurring moisture in the atmosphere, which can affect the properties of the fluid. For this reason, the use of fluids with no more than 0.5% water absorption and, in particular, less than 0.1% is preferred according to the invention.
While the known use of water in density determinations presents no problem in regard to environmental compatibility or toxicity, there can be certain problems in this regard when using another fluid. It is therefore advantageous to use a fluid that contains no fluorocarbon chlorides and is non-toxic.
Another obvious problem that could occur with a fluid other than water is flammability. It is a particular advantage of the invention that fluids with the aforementioned characteristics are available that have flash points above 50xc2x0 C. and self-ignition points above 350xc2x0 C., i.e., fluids that are non-critical with regard to flammability.
In the experiments and tests performed by the inventor(s), it was found that excellent results, at least by an order of magnitude more accurate, are obtainable by using a fluid containing at least one silicon hydride. Silicon hydrides, also called silanes according to IUPAC rule D-6.14h, include branched as well as unbranched silicon hydrides. Substitutions such as, e.g., silyls, are also entirely within the scope of the invention. Particularly preferred are cyclosilanes as they meet most of the required characteristics mentioned above, for example alcoholic cyclosilanes in which at least one alcohol group such as an ethyl-, methyl-, buthyl-, propyl-, iso-propyl alcohol or the like is attached to the silicon atom. Of course, these or similar alcohols can be substituted by esters (generally of the low-valence kind), in fewer cases also by ether. Particularly preferred are fluids in which the alcohol group attached to the silicon atom is a methyl group.
Cyclosilanes with at least four silicon atoms, i.e., relatively large ring formations, are preferred. In these formations, it is preferable to have at least two alcohol groups attached to each silicon atom, particularly in a branched configuration.
In practical applications, octamethyl-cyclotetrasiloxane and higher-order ring formations such as dekamethyl-cyclopentasiloxane have been proven to produce excellent results. The surfactant properties (surface tension of 18 to 19 N/m at 25xc2x0 C.) have turned out to be a particularly favorable trait of these compounds in that they prevent the formationxe2x80x94or promote the rapid disappearancexe2x80x94of gas bubbles that cling to the surface of the solid body being tested and are detrimental to the accuracy of the measurement. It should be noted that cyclo-silanes of this kind are commercially available for different applications, e.g., as coolants, detergents, and solvents, and that they have the following distinctive combination of properties: Their density is less than 1 g/cm3; they have a low toxicity, allowing them to be sold without restriction; their self-ignition temperature is 390-400xc2x0 C. and higher; their flash point is between 50 and 80xc2x0 C.; their hygroscopicity (water absorption) does not exceed 0.1%; their vapor pressure is lower than for water by a factor of 4; they generally maintain their physical properties over time at different temperatures; and they have good wetting properties (even to the extent of eliminating the need for the degassing step that is common with water). Many of the fluids that contain cyclosilanes of these kinds are nearly azeotropic mixtures.